1. Field of the Invention
This invention relates generally to the field of thermal barrier coatings, and more particularly to a thermal barrier coating for a very high temperature application such as a combustion turbine engine. In particular, this invention relates to the field of multiphase ceramic thermal barrier coatings for high temperature application for coating superalloy components of a combustion turbine.
2. Background Information
The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher firing temperatures in the combustion portions of these systems. Although nickel and cobalt based "superalloy" materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes exceeding 1,200.degree. C.
Examples of cobalt or nickel based superalloys are, for example, Cr.Al.Co.Ta.Mo.W which has been used for making SC turbine blades and vanes for gas turbines, as taught, for example, in U.S. Pat. No. 5,716,720 (Murphy).
These turbine components are generally protected by a basecoat of MCrA1Y, where M is selected from the group of Fe, Co, Ni, and their mixtures, as taught for example, by U.S. Pat. Nos. 5,763,107 and 5,846,605 (both Rickerby et al.) and by U.S. Pat. Nos. 4,916,022; 5,238,752; 5,562,998; and 5,683,825 (Solfest et al.; Duderstadt et al.; Strangman; and Bruce et al., respectively). These basecoats are usually covered by an aluminum oxide layer and a final thermal barrier coating ("TBC"). The standard thermal barrier coating, however, is made from yttria-stabilized zirconia, ceria-stabilized zirconia, scandia-stabilized zirconia or non-stabilized zirconia, as taught, for example, by U. S. Pat. No. 5,780,110 (Schaeffer et al.). A particularly useful state of the art TBC is 8 wt. % yttria stabilized zirconia ("8YSZ").
Many of the ceramic thermal barrier layers are deposited as a columnar structure in the direction of the coating layer thickness, as taught in Murphy U.S. Pat. No. 5,716,720. This structure can be formed by plasma assisted physical vapor deposition or electron beam physical vapor deposition, as taught, for example, in Rickerby et al. U.S. Pat. No. 5,846,605 and in Murphy U.S. Pat. No. 5,716,720, respectively.
Another type of ceramic thermal barrier is deposited as a flat layer parallel to the surface of the superalloy structure, and applied, for example, by plasma spraying techniques, as taught in U.S. Pat. 5,824,423 (Maxwell et al.). The Rickerby et al., U.S. Pat. No. 5,846,605 characterizes these type of coatings as having a "brick wall" microstructure which has poor erosion resistance but a lower thermal conductivity than columnar structures.
Much of the development in this field of technology has been driven by the aircraft engine industry, where turbine engines are required to operate at high temperatures, and are also subjected to frequent temperature transients as the power level of the engine is varied. A combustion turbine engine installed in a land-based power generating plant is also subjected to high operating temperatures and temperature transients, but it may also be required to operate at full power and at its highest temperatures for very long periods of time, such as for days or even weeks at a time. Prior art insulating systems are susceptible to degradation under such conditions at the elevated temperatures demanded in the most modern combustion turbine systems.
Modern gas turbine engines can achieve higher efficiencies by increasing the turbine inlet temperatures. This subjects the TBCs to high temperatures. TBC materials that are phase stable at high temperatures upon long term exposure will be required. The current state-of-the-art air plasma sprayed ("APS") 8YSZ coatings destabilize above approximately 1200.degree. C. In addition, the long term high temperature exposure leads to potential sintering and loss of strain compliance, and possible premature TBC failure. 8YSZ coatings are also susceptible to corrosion upon exposure to contaminants in the fuel and erosion due to foreign object damage. Therefore, some of the key requirements for new TBC candidates for high temperature applications are high temperature phase stability, a reduced tendency to sinter, good corrosion and erosion resistance, all of them to be maintained upon long term exposure. These requirements are in addition to the primary needs of a TBC, such as, a low thermal conductivity with minimal coefficient of thermal expansion mismatch with the superalloy substrate. Further advances in gas turbine operating temperatures therefore require a plasma sprayed ceramic thermal barrier coating capable of surface temperature in excess of 1200.degree. C.